fix: fixes QueryParser parsing of dasherized/camelized filter keys

[grzuy]

Analogous to #431 but for filter.

QueryParser is already automatically underscoring fields, sort and include, just missing filter.

Seems this would be the last fix needed to "close" Standardize on underscoring query parameters #285.

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Underscoring of filter keys is technically already supported by UnderscoreParameters plug, but it feels that this should work for anyone using QueryParser and not UnderscoreParameters plug.

I kind of agree with the following statement by @TylerPachal statement in #285

I think for version 2.0 of this library:

All of the query parameters should always be underscored. It should be done by the QueryParser module.
The UnderscoreParameters module should go back to just doing the params, mostly undoing the work in #282.